You've asked a number of good questions, which indicates that you're doing your homework before buying.

The mileage is what it is. 22k miles on an early '08 is not ideal, but, it's not terrible either. The fact that it was a leased vehicle (and a corporate lease at that) could be cause for some concern if the owner did not maintain the vehicle properly and/or if it was abused. However, at the end of the day, there's no way to know for sure how the car was treated.

You didn't mention whether the dealer selling the vehicle is a BMW dealer. If the vehicle is being sold by a BMW dealer, you have little to worry about. A BMW dealer will not put a BMW on their lot that is known to have issues (CPO or not). Not being a CPO vehicle (at a BMW dealership) is not necessarily an issue either. It costs the dealer quite a bit of money to classify a BMW as a CPO vehicle. The dealer likely assessed the vehicle and determined that it's clean enough to not warrant the additional investment. That doesn't mean that it's perfect; as would be expected with any used vehicle. Not being a CPO would be especially likely on an M car as they don't need incentives to be sold quickly.

If the vehicle is not being sold by a BMW dealer, then you have some additional work to do. Again, there's no way to know for certain how the vehicle was treated. Additionally, you should not rely on CARFAX or AutoCheck for the vehicle maintenance history. The only way to properly assess the vehicle's condition and to determine the maintenance history is to have the car inspected by a BMW dealer. Any reputable dealer will allow you to take their car to an independent source (like a BMW dealer) to be inspected (at your cost, of course). BMW performs such inspections routinely. They will also provide you with the maintenance history on the vehicle. If the maintenance history on the vehicle is deficient in BMW's system, it can be brought current after you purchase the car. There also might be a reasonable explanation for the deficient history, such as, the owner lived too far from a BMW dealer and had the maintenance performed locally. In short, all potential issues will be uncovered during a BMW inspection. And, if there are any issues, your car is still under warranty.

Regarding the early build date, I wouldn't worry too much about that. There are very few known issues with the vehicle. The only major issue (relatively speaking) is with the DCT (if so equipped). There is a TSB (Technical Service Bulletin) on the DCT that requires an ECU re-flash. Review of the service history will indicate if the TSB has been performed. That service is of course performed at no cost to you.

Tail light issues have been reported by some owners on this forum. Multiple bulb replacements would tend to indicate a socket or wiring problem. I don't have a lot of info on that issue, but, it wouldn't stop me from buying an M3!

In summary:

1. Being sold be a BMW dealer? No worries.

2. Not being sold by a BMW dealer? Have the vehicle inspected by BMW.

That's about all I have for you. I've typed too much already. Hope the info helps!